Based on the above map, the Apostle John from Patmos, could have seen flying lava rocks calling it hail. Wikipedia lists ten volcanoes in Greece.

​5-4-18, Scripture speaks of signs in the heavens (Luke 21:25-26) and Revelation 16:21 tells of 100 pound hail ... volcanoes hurl chunks of lava that cools and turns into rocks flying through the air. Many people think this 'hail' is like the icey chunks we see in a winter storm. But what if this pumice, debris, magma rocks and ash that travels miles from the sky actually originates from volcanoes ... and isn't ice? [The size and weight of the rocks from volcanoes: 1 cu ft of lava rocks weighs 50 lbs: measuring 1 ft by 1 ft by 1 ft by 1 ft.]

Joel 2:2-5, 30-31 fire and smoke sun darkened moon turned to blood. Sure sounds like the affects from an erupting volcano; also the smoke and ash block the sun [1a] (Isaiah 13:10).

Isaiah 64:1-3"Oh that you would rend the heavens, that you would come down, that the mountains might flow down at your presence,"

Micah 1:4"And the mountains shall be molten under him, and the valleys shall be cleft, as wax before the fire, and as the waters that are poured down a steep place."

Acts 2:20"The sun shall be turned into darkness, and the moon into blood, before that great and notable day of the Lord come:"

Revelation 6:14"And the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places."

[1] Distance rocks flies from volcano: Highlights from the 1883 Krakatau eruption ash and rocks traveled as follows: Singapore 521 miles to the north, Cocos (Keeling) Island 717 miles to the SW and ships as far as 3775 miles west-northwest.

[1a]Darkness covered the Sunda Straits from 11 a.m. on the 27th until dawn the next day.http://volcano.oregonstate.edu/how-high-can-explosive-eruptions-go-and-how-far-can-debris-and-ash-be-spread